Methods and apparatus for heart failure treatment

ABSTRACT

Methods and apparatus for assessing the condition of and treating patients for heart failure by the delivery of continuous positive airway pressure are disclosed. Treatment of obstruction due to reflex vocal cord closure often experienced by heart failure patients is distinguished from treatment of upper airway obstruction typically associated with Obstructive Sleep Disorder. Treatment may also be implemented by delivering synchronized cardiac pressure oscillations superimposed on a respiratory pressure level to provide assistance for the heart. Heart treatment pressure dose indicator may be calculated for prescribing and monitoring the delivery of treatment. The apparatus may also generate data to track heart failure condition that may be indicative of the degree of severity of heart failure based upon breathing patterns to assist in the diagnosis and management of heart failure patients.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of U.S. patent applicationSer. No. 12/613,035, filed Nov. 5, 2009 which is a continuation of U.S.patent application Ser. No. 10/575,197, filed Jul. 31, 2006, which is acontinuation of International Application No. PCT/AU04/01420, filed Oct.15, 2004, which claims the benefit of the filing date of U.S.Provisional Patent Application No. 60/512,553, filed on Oct. 17, 2003,all of which are hereby incorporated herein by reference.

FIELD OF THE INVENTION

The invention relates to methods and apparatus for diagnosing, managingand treating congestive heart failure.

BACKGROUND OF THE INVENTION

It has been estimated that in the United States alone that almost fivemillion people suffer from congestive heart failure. Statistics from theAmerican Heart Association also suggest that new cases of heart failureare diagnosed at a rate of about 500,000 each year. Of the newlydiagnosed patients fifty percent are likely to die within five yearsfrom the initial diagnosis. Of course, these numbers do not account forthe number of patients in other countries who also suffer from heartfailure. Given these numbers, it is clear that congestive heart failureis a significant human crisis.

Heart failure is a condition that is characterized by a reduced abilityof the heart to circulate blood through the body. Typically, anunderlying disease, such as high blood pressure (e.g., hypertension),clogged arteries (e.g., coronary artery disease), heart defect (e.g.,cardiomyopathy, or valvular heart disease) or some other problem (e.g.,diabetes, hyperthyroidism, or alcohol abuse) will lead to a decrease incirculation over time. As the heart works less efficiently, its capacityto circulate blood decreases and the body's requirements for oxygen arenot met. The cardiac muscle tends to enlarge as the heart works harderover time to compensate for the decrease in efficiency.

Heart failure may be identified by the phase of the heart cycle that isparticularly associated with the nature of the circulatory problem. Bythis identification, two types of heart failure are known as systolicand diastolic. In systolic heart failure, the cardiac muscles' abilityto contract decreases. This loss of contraction results in a decrease inthe ability of the heart to force blood through the circulatory systemof the body. In diastolic heart failure, the cardiac muscles' ability torelax diminishes. As the heart muscles become stiffer, the heart doesnot sufficiently fill with blood and thus each subsequent contractioncirculates a lower volume of blood.

Alternatively, heart failure may be characterized by whether it stemsfrom problems with the left or right side of the heart. Left-sided heartfailure occurs when the left ventricle does not sufficiently pumpoxygenated blood to the body. Right-sided heart failure occurs when theright ventricle does not pump adequately, which leads to fluid build-upin the veins.

Although the phrase “congestive heart failure” is often used to describeall types of heart failure including the above listed types, congestiveheart failure is more accurately descriptive of a symptom of heartfailure relating to pulmonary congestion or fluid buildup in the lungs.This congestion is more commonly symptom of systolic and left-sidedheart failure. As the efficiency of the pulmonary system declines,increased blood volume near the input side of the heart changes thepressure at the alveolar arterial interface, an interface between thelung capillaries and the alveolar space of the lungs. The change inpressure at the interface causes blood plasma to push out into thealveolar space in the lungs. Shortness of breath (“dyspnea”) and generalfatigue are typical perceived manifestations of congestive heartfailure.

Congestive heart failure (“CHF”) is currently classified by severity.Class I patients have no apparent symptoms and no physical activitylimitations. Class II patients experience some symptoms during moderateto severe physical activity. Class III patients suffer symptoms at mildlevels of physical activity. Class IV patients experience symptoms withany form of physical activity as well as at rest.

While the only cure to CHF is heart transplant, there are a number ofdrug and surgical treatments directed at reducing the underlying problemthat led to the heart failure and/or to alleviate the symptoms of heartfailure. For example, the use of diaretics is intended to reducecongestion by depleting the body of fluids. Vasodilators such as ACEinhibitors are used to expand blood vessels and reduce resistance toblood circulation. Beta blockers seek to reduce the work load on theheart by normalizing the rhythm of the heart. Cardiotonic drugs aredirected at increasing the force of the heart's contractions. Surgicalprocedures include physical manipulation in an attempt to increase theinternal size of constricted arteries, for example, by balloonangioplasty or stenting.

As previously noted, as a consequence of heart failure there is adecreased flow of oxygen in the circulatory system. This decease inoxygenated blood through the body has an impact on the body'srespiratory controller. Thus, there appears to be a relationship betweencongestive heart failure and respiratory conditions known as SleepDisordered Breathing (“SDB”). For example, it has been noted that 50-60%of heart failure patients have SDB. In this category of patients,approximately 29% may be classified as suffering from obstructive sleepapnea, a breathing condition associated with the cessation or limitationof airflow due to occlusion usually at the level of the tongue or softpalate. In addition, 33% of the patients maybe classified as sufferingfrom (a) Cheyne-Stokes respiration, a breathing condition characterizedby waxing and waning breathing patterns or (b) central sleep apnea, acondition involving a cessation of airflow due to a cessation of patientrespiratory effort. For those patients suffering from Cheyne-Stokesbreathing, there is a greater degree of concern. These patients have ahigher mortality rate then heart failure patients without Cheyne-Stokesbreathing.

Sleep disordered breathing has long been treated by application ofContinuous Positive Airway Pressure (“CPAP”). CPAP was invented bySullivan and taught in U.S. Pat. No. 4,944,310. That patent describescontinuous positive airway pressure being applied to a patient, throughthe patient's nares, to treat breathing disorders, including obstructivesleep apnea. It has been found that the application of pressure whichexceeds atmospheric pressure, typically in the range 4 to 15 centimetersof H₂0, is useful in treatment. The pressure acts as a pneumatic splintto maintain upper airway patency to ensure free flow of air while thepatient sleeps.

In one form, nasal CPAP treatment of Obstructive Sleep Apnea (“OSA”)involves the use of an automated blower, such as the AUTOSET T™ deviceor AUTOSET SPIRIT™ available from ResMed Ltd., to provide a constantsupply of air or breathable gas at pressures in the range of 4 to 20 cmH₂O to the airway of a patient via a mask. Examples of suitable nasalCPAP masks are the MIRAGE™ nasal mask and the MIRAGE™ full face maskalso available from ResMed Ltd. The AUTOSET T™ device continuouslymonitors the state of the patient's airway and determines an appropriatepressure to treat the patient, increasing it or decreasing it asnecessary. Alternatively, bilevel pressures are delivered to the patientas in the VPAP II™ devices also available from ResMed Ltd. Some of theprinciples behind the operation of the AUTOSET T™ and VPAP II™ devicesare described in U.S. Pat. No. 5,704,345. The entire disclosure of U.S.Pat. No. 5,704,345 is incorporated herein by reference.

One form of pressure treatment is delivered in accordance with a smoothpressure waveform template and a continuous phase variable to providecomfortable pressure support substantially in phase with the patient'srespiratory cycle. The device is the subject of InternationalPublication No. WO 98/12965. The device is also the subject of U.S.patent application Ser. No. 08/935,785, now U.S. Pat. No. 6,532,957, theentire disclosure of which is hereby incorporated by reference.

Another form of pressure treatment is directed at treatment ofCheyne-Stokes breathing. In a device designated AUTOSET CS™, alsoprovided by ResMed Ltd., pressure support is varied in phase withpatient respiration in such a manner to oppose the waxing and waningchanges in patient respiration that characterize Cheyne-Stokesbreathing. The device is the subject of International Publication No. WO99/61088. The device is also the subject of a U.S. patent applicationSer. No. 09/316,432, now U.S. Pat. No. 6,532,959, the entire disclosureof which is incorporated herein by reference.

At present, there are no known devices with features designed to treat arange of symptoms of heart failure patients through application ofpressure as opposed to devices that might only incidentally provide suchbenefits. U.S. Pat. No. 5,794,615 teaches a device to provide a level ofpressure support to reduce cardiac pre-load and after load. However, thedevice is only taught to provide one continuous level of pressure duringinspiration and another level during expiration. The patent does notsuggest the provision of a waveform of cardiac pressure oscillations inphase with a patient's cardiac rhythm, a feature of the presentinvention as described below. Moreover, the device provides noassistance directed to alleviating Cheyne-Stokes breathing ordistinguishing between obstructions due to vocal cord reflex andobstructions from typical OSA due to collapse of the upper airway duringsleep.

Any reference herein to known prior art does not, unless the contraryindication appears, constitute an admission that such prior art iscommonly known by those skilled in the art to which the inventionrelates, at the priority date of this application.

BRIEF SUMMARY OF THE INVENTION

It is an objective of the invention to provide methods and apparatus formanaging the treatment of respiratory disorders in congestive heartfailure patients.

It is a further objective to provide methods and apparatus that assistin the identification or diagnosis of heart failure to assist withtreatment of the patient.

Other objectives will be apparent to those skilled in the art from areview of the description of the invention as contained herein.

The invention provides methods and apparatus for detecting reflex vocalcord closure. The vocal cord closure detector derives a measureindicative of the closure. Preferably, the measure is indicative of astate of sleep and may be derived from respiratory airflow of thepatient as a function of a minute ventilation and an elapsed time. Thedelivery of positive airway pressure treatment is controlled as afunction of the measure. In the preferred embodiment of the invention,any apnea or obstructive event detected before about 30 minutes of sleepis determined to be vocal cord closure and treatment levels are notincreased. Apneas detected after these thresholds are met and treated asa non-vocal cord obstructive apnea by an increase in pressure.Alternatively, reflex vocal cord closure may be detected bydistinguishing an incident of vocal cord closure from another type ofairway obstruction based on a derived measure indicative of vocal cordclosure. The step of distinguishing may include detecting an obstructiveevent and conditioning an increase in treatment pressure in response tothe detected obstructive event by an analysis of the derived measure ofthe closure. This analysis may include a comparison of the derivedmeasure with a time limit of about 30 minutes. The invention furtherincludes methods and apparatus for providing a synchronized cardiacwaveform to perform some work of the cardiac organ. The cardiac waveformmay be a square wave or sinusoidal wave in phase with detected cardiacrhythm and is preferably superimposed with continuous, hi-level or otheroscillatory respiratory treatment pressure that supports the patient'srespiration or maintains an open airway. The cardiac waveform ispreferably delivered with amplitudes in a range of approximately 1 to 2cm H₂O.

The invention also includes a means for calculating a heart treatmentindex to regulate or measure the dose of treatment to the heart from thesynchronized oscillations. The measure determines the index as afunction of duration and delivered pressure and preferably accounts onlyfor time that the treatment oscillations are actually delivered to thethorax by excluding treatment during closed airway apnea or periods ofhigh leak. In one embodiment of the invention, the index is the productof an average pressure and the duration of treatment.

Finally, the invention includes methods and apparatus for assessingheart failure in a patient by calculation or determination of a heartfailure indicator or index. Such an indicator may be determined fromrespiratory airflow by assessing an extent of Cheyne-Stokes breathing inthe patient. Alternative embodiments of the indicator include measuresof the duration of waxing and waning cycles or frequency analysis ofcomponents of a measure of airflow in a range of frequencies associatedwith Cheyne-Stokes breathing. In addition, an appropriate indicator maybe a measure of minute ventilation compared with a threshold of about 15L/min. or alternatively a ratio of a minimum and maximum of a measure ofventilation, such as a minute ventilation or a tidal volume. Changes insuch indicators taken by comparing or analyzing indicators from acurrent session with assigned thresholds or predetermined thresholdvalues including indicators from prior sessions provide a diagnostictool for assessing improvement or deterioration of the patient'scondition.

BRIEF DESCRIPTION OF THE DRAWINGS

An embodiment or embodiments of the present invention will now bedescribed, by way of example only, with reference to the accompanyingdrawings, in which:

FIG. 1 shows apparatus according to the invention;

FIG. 2 is a flow chart of an embodiment of the invention for detectingvocal cord closure;

FIG. 3 is a flow chart of an embodiment of the invention for deliveringtreatment based on the detection of vocal cord closure;

FIG. 4 is a graph of one form of a synchronized cardiac pressureoscillations in accordance with the invention;

FIG. 5 is a graph of a respiratory treatment pressure waveform inaccordance with the invention;

FIG. 6 is a graph of superimposed cardiac pressure oscillations withrespiratory treatment pressure;

FIG. 7 depicts several graphs relating to detection of Cheyne-Stokesbreathing in a patient;

FIG. 8A depicts a graph of minute ventilation determined from a flowsignal from a patient experiencing Cheyne Stokes breathing and a graphof a frequency spectrum of the minute ventilation;

FIG. 8B depicts a graph of minute ventilation from a patientexperiencing normal breathing and a graph of a frequency spectrum fromthe minute ventilation;

FIG. 9 illustrates a treatment protocol for distinguishing between vocalcord closure and upper airway obstruction;

FIG. 10 illustrates a method of detecting obstruction from a measure ofpressure and ventilation;

FIG. 11 is a flow chart illustrating steps in a methodology fordetermining a positive pressure dose measure of the invention;

FIG. 12 is a flow chart illustrating steps in a methodology fordetermining a heart failure indicator or index.

DETAILED DESCRIPTION

In reference to FIG. 1, the heart failure treatment invention involvesan apparatus that includes a blower 2, a flow sensor 4 f, pressuresensor 4 p, a mask 6, and an air delivery conduit 8 for connectionbetween the blower 2 and the mask 6. Exhaust gas is vented via exhaust13. Mask flow is preferably measured using a pneumotachograph anddifferential pressure transducer to derive a flow signal F(t). Maskpressure is preferably measured at a pressure tap using a pressuretransducer to derive a pressure signal Pmask(t). The pressure sensor 4 fand flow sensor 4 p have only been shown symbolically in FIG. 1 since itis understood that those skilled in the art would understand how tomeasure flow and pressure. Flow F(t) and pressure Pmask(t) signals aresent to a controller or microprocessor 15 to derive a pressure requestsignal PRequest(t). The controller or processor is configured toimplement the methodology described in more detail herein and mayinclude integrated chips, a memory and/or other instruction or datastorage medium. Programmed instructions may be either coded onintegrated chips in the memory of the device or may be loaded assoftware.

The apparatus further includes a communication port or module 10, forexample, a wireless communication transceiver and/or a network card, forcommunication with other devices or computers such as hand-held displayand control devices 12. The apparatus optionally includes an oximeter inthe main blower housing. There is a sense tube 14 connected to the mainhousing of the blower to the mask that allows the apparatus to senseoxygen concentration and pressure levels in the mask 6. The apparatusmay further include additional communications interface 16 forconnection to additional diagnosis devices. The diagnosis unitoptionally includes a pulse oximeter 20, respiratory movement sensors22, EEG & ECG 24 and/or EOG 25. The unit may also include a set ofelectrodes 28 for detecting cardiac rhythm.

While this apparatus is described as a single unit, it is understoodthat a combination of devices and/or computers linked by any availablecommunications method may be used to accomplish the goals of theinvention. For example, the apparatus can interface with a variety ofhand-held devices such as a Palm Pilot via wireless communication. Withsuch a device, a physician may, for example, remotely monitor, analyzeor record the status or data history of a patient or diagnose theseverity of the patient's condition using the device. For example,remote devices may store heart failure indicators, such as in a databaseof patient heart failure recovery information for one or more patients,from data generated by use of the apparatus. Furthermore, the treatmentprogram that is being run on the patient can be monitored and changedremotely. In the event patient data is transmitted over open networks,the data may be encrypted for purposes Of patient confidentiality.

The apparatus incorporates various treatment protocols. One protocol isintended for treating obstructive apneas. Another is for treatingcentral apneas. An additional protocol is for treating Cheyne-Stokesbreathing. As described in more detail below, the apparatus determinestreatment automatically.

In one mode, the device provides a generally constant pressurethroughout a breathing cycle, but may vary the pressure in accordancewith indications of partial or complete obstruction of the airway. Onetechnique for accomplishing this using a combination of flow limitationand snore measurements is described in U.S. Pat. No. 5,704,345(Berthon-Jones). Other known alternative methods to vary the pressurefor delivering CPAP treatment to a patient to treat obstructive apneaswould be recognized by those skilled in the art and may be utilized asoperating modes in the device.

In another mode, the apparatus provides a higher pressure to the maskduring the inspiratory portion of the breathing cycle, a so-called IPAP(inspiratory positive airway pressure), and a lower pressure to the maskduring the expiratory portion of the breathing cycle, a so-called EPAP(expiratory positive airway pressure). This may be accomplished bymonitoring the respiratory flow to the patient and defining thresholdlevels to distinguish between inspiration and expiration. When flowexceeds a threshold then the device delivers IPAP. Below a threshold,the device delivers EPAP.

Alternatively, the treatment delivered by the apparatus will smoothlyvary in accordance with patient respiration to provide a smooth pressurewaveform. For example, the device calculates a continuous phase variableto provide support in phase with the patient's breathing cycle andcalculates the pressure to be delivered in accordance with a pressurewaveform template. The delivery of such pressure is disclosed in U.S.patent application Ser. No. 08/935,785. Alternatively, pressure may besupplied in proportion to patient respiratory airflow.

In another form, pressure support is varied in phase with patientrespiration in such a manner to oppose the waxing and waning changes inpatient respiration that characterize Cheyne-Stokes breathing. Themethodology for such treatment is disclosed in U.S. patent applicationSer. No. 09/316,432.

While the blower 2 may alternately generate different pressure levels inaccordance with the varying pressure delivery methods just described, inan alternative version, a near-constant speed of blower 2 can bemaintained and the pressure drops are achieved by venting with theinclusion of a controllable release valve. The same apparatus can beused for many different therapies simply by adjusting the equation thatis used to set the speed of the blower or to manipulate the venting withthe release valve.

In providing these treatment methodologies, an accurate determination ofrespiratory airflow is important. Thus, the flow rate of air to thepatient is adjusted to account for the effect of leak. To this end, leakairflow may be determined by using a method such as taught in U.S. Pat.No. 6,152,129 (Berthon-Jones), the entire disclosure of which isincorporated herein by reference. Other known methods for determiningleak may also be used by the device.

With such a device, positive pressure ventilation can be applied in thetreatment of heart failure patients as further described herein.Positive pressure addresses the symptoms of heart failure patients by(a) providing increased airflow to assist in drying fluid from thelungs; (b) reducing fluid transfer to the lungs by increasing thepressure in the alveolar space to offset the pressure differentialacross the alveolar arterial interface; (c) performing some work of theheart to assist with circulation by reducing the size of the heart toallow the heart to operate more efficiently as a result of increasedpressure in the thoracic cavity adjacent to the heart or by providing acontracting assistance or oscillating force in the thoracic cavity; (d)supporting respiration to provide ventilatory assistance thatcompensates or prevents the waxing and waning cycles associated withCheyne-Stokes breathing while also providing support to prevent or treatobstructive events; and (e) performs some portion of the work ofbreathing. Further objectives will be apparent to those skilled in theart based upon the disclosure herein.

A. Reflex Vocal Cord Closure Detection

One of the complexities of the design of the operation of such a devicerelates to the nature of the breathing difficulties experienced by CHFpatients. As previously noted, CHF patients are likely to experienceCheyne-Stokes breathing, central apneas and/or obstructive events.However, the treatment protocol for each may be distinct. Therefore, adevice of the invention is configured to automate a change of thetreatment protocol based upon the likelihood of the occurrence of thevarious respiratory and airway abnormalities particularly associatedwith heart failure patients.

To this end, it has been observed that CHF patients may experienceCheyne-Stokes breathing while a patient is awake. Patients may alsoexperience Cheyne-Stokes breathing or central apneas in earlier stagesof sleep particularly stage 1 and stage 2 sleep but not typically duringREM sleep. Patients also experience obstructive apneas due to upperairway collapse. Such collapse is typically a result of the relaxedstate of the patient induced by sleep. Therefore, these obstructiveapneas may occur during the latter stages of sleep and are more likelyto occur during REM sleep.

Methods for the detection of obstructive apnea, airway obstruction andcentral apnea are disclosed in detail in U.S. Pat. No. 5,704,345 and areotherwise known in the art. Obstructive events may be determined byanalysis of patient flow to determine shape factors, flow flatteningindices, roundness indices, etc. Moreover, with a detected apnea, (e.g.,a calculated variance falling below a threshold value) it can bedetermined whether the apnea constitutes airway obstruction or anabsence of respiratory effort (i.e., central apnea). In one suchtechnique, when an apnea is detected as occurring, the apparatus appliesan oscillatory pressure waveform of known frequency and magnitude andassesses the patency of the airway from the flow that is induced in theairway. In one form, if the airway is patent during an apnea, then theapnea is judged to be central. However, if the airway is closed duringan apnea, then the apnea is judged to be obstructive. In anothertechnique, when an apnea is detected as occurring, the apparatusmonitors the airflow for the presence of a signal of cardiac origin. Ifa cardiac signal is detected, then the airway is judged to be patent andthe apnea classified as central. If no cardiac signal is detected, thenthe airway is judged to be closed and the apnea classified asobstructive.

Other methods for distinguishing between central and obstructive apneasinclude monitoring chest movement to detect physical respiratory effortusing respiratory bands or monitoring the movement of the suprasternalnotch, for example, as taught in International Patent Application WO01/19433 (Berthon-Jones et al.), also taught in U.S. patent applicationSer. No. 08/396,031, the disclosure of which is hereby incorporated byreference. When there is no respiratory effort when an apnea isdetected, it may be considered a central apnea rather than obstructive.

An alternative method for determining the existence of obstructiveevents involves an analysis of the relationship between a measure ofventilation and changes in pressure support. For example, if a measureof minute ventilation does not increase or remains the same when supportpressure is increased, this would indicate that the patient isexperiencing an obstructive event. Thus, the failure of the measure ofventilation to increase in relation to increases in pressure supportwould tend to indicate that the patient's airway is obstructed. In sucha methodology, the measure of minute ventilation is monitored to detecta decrease in the minute ventilation. When pressure ventilation isincreased to compensate for the decrease in the measure of minuteventilation, if the measure of minute ventilation does not increase inthis general time frame, the device interprets the condition asdetecting an incident of obstruction. This method is illustrated in thegraphs of FIG. 10. The graphs plot a continuously determined minuteventilation (e.g., the volume of air inspired by the patient during theprevious 60 seconds) and pressure delivered at the mask with respect toa common time scale. An obstructive event is determined when, after adecrease in minute ventilation shown at 100, there is no increase inminute ventilation corresponding to an increase in pressure 102. Whilethe graph illustrates the method during delivery of a relativelyconstant CPAP pressure, those skilled in the art will recognize that themethod may be accomplished in the presence of bi-level treatment orother pressure support which synchronizes smooth and comfortablepressure changes with the patients respiratory cycle by monitoringchanges in end expiratory pressure.

Heart failure patients suffering from Cheyne-Stokes breathing are likelyto suffer from reflex vocal cord closure in the initial stages of sleepwhen PCO 2 in the blood is low, approximately between 5 and 30 minutesinto sleep. As stable sleep is entered and partial pressure of carbondioxide (PCO2) increases, the likelihood of these events diminishes.Vocal cord closure may generally be considered an obstructive event thatmay be detected in the manner that upper airway occlusions at the levelof the tongue or soft palate are detected. Existing methods ofobstruction detection as previously mentioned will detect both upperairway collapse and vocal cord closure but they cannot distinguishbetween these events. Due to the dangerous and counter-productive levelsof pressure that would be required to open vocal cord closure (about 60to 70 cm H₂O or higher), the vocal cord event preferably is not treatedlike that of typical obstructive apneas, i.e., by increasing pressure,such as the end expiratory pressure, until the obstruction is opened.While vocal cord events may be treated by the same levels of pressure asother obstructive events, the treatment levels would not likely open theclosure. Rather, such treatment is only likely to disturb the patient'ssleep and prevent development of more stable deeper sleep. For thesereasons, no increase in the treatment of reflex vocal cord closure ispreferred.

Accordingly, in determining the appropriate treatment protocol, a deviceof the invention preferably estimates or approximates whether thepatient is awake or asleep, e.g., in some stage of sleep, and thusdistinguishes between vocal cord closure and more typical obstructiveevents associated with sleep apnea. General steps in such a methodologyare summarized in the flow chart of FIG. 2. In a delivering step 30, acontrolled supply of breathable gas at a pressure above atmospheric issupplied to the patient. In a deriving step 32, a flow derived measureindicative of a vocal cord closure in a patient is determined. In adetecting step 34, an incident of vocal cord closure is detected as afunction of the indicator.

With such an indicator, the device then selects between differenttreatment regimes. For example, initially, Cheyne-Stokes breathing andcentral apneas are treated while the patient is awake or in the earlystages of sleep by delivering variations in pressure in phase withpatient respiration to meet a target ventilation. During this treatmentperiod obstructive events are ignored. Alternatively, obstructive eventsmay be detected by observing an absence of or substantial decrease inairflow but those obstructions that are likely to be vocal cord closureare preferably not treated according to the indicator. After satisfyinga threshold comparison with the indicator, subsequent obstructive eventsthat are detected will be treated by an increase in pressure, such as,by increasing end expiratory pressure. Such a methodology is illustratedin the flow chart of FIG. 3. In a detection step 36, airway obstructionis detected. In an evaluation step 38, an indicator of vocal cordclosure is determined. Finally, in a treatment step 40, treatment isdetermined as a function of the indicator. If vocal cord closure isdetected then pressure is maintained at the current level or decreased.If vocal cord closure is not detected then pressure is increased as inthe case of a typical obstruction. Appropriate pressure responses totypical obstructive events is described in more detail in U.S. Pat. No.5,704,345.

One such indicator relates to a measure of ventilation, for example, aminute ventilation, i.e., the volume of measured airflow over the courseof a minute. When a ventilation measure falls below a certain threshold,the indicator may suggest that the patient is in a later stage of sleep.To this extent, it will serve as an indicator to distinguish betweenobstructive events of vocal cord closure as opposed to tongue and softpalate closure. For example, if the minute ventilation, preferablyaveraged over a period of time, e.g., five minutes, is in a range ofabout 5 to 10 liters per minute, the patient is likely in a later stageof sleep. The accuracy of the indicator may be improved by making thethreshold determination an additional function of time. For example, ifthe measure of ventilation is below a certain level and a period of timehas elapsed, such as about 30 minutes, the patient is more likely to bein a later stage of sleep. In the preferred embodiment, if the mask hasbeen on the patient for more than about 30 minutes and the minuteventilation averaged over a period of about five minutes is less thanabout 12 liters per minute, a later stage of sleep is indicated. Thiswould also indicate that any detected obstructive events are of the moretypical upper airway collapse of traditional obstructive sleep apnearather than reflex vocal cord closure.

Another alternative indicator may be based upon the pressure swing.Swing is the difference between inspiration and expiration pressurelevels as delivered by the apparatus in keeping with the effort of thepatient's respiratory cycle. Typically, pressure swing is in the rangeof about 3 to 10 cm H₂O. Higher pressure swings in the range near about10 cm H₂0 may be indicative of Cheyne-Stokes breathing. Thus, if swingis lower than that range for a period of time, e.g. about ten minutes,it would tend to indicate that the patient has settled into a laterstage of sleep. Therefore, as the swing stabilizes, i.e., approaches athreshold, for example, about 8 cm H₂0 or less, and maintains thatthreshold for a certain time, the swing will be indicative of a laterstage of sleep.

In a more simplified embodiment, a measure of time may serve to detectvocal cord closure by distinguishing between obstructive apneas andvocal cord closure based on this measure. In this embodiment, anydetected obstructive events prior to the expiration of a period of timeare considered to be vocal cord closure and not treated. For example,for any obstructive event detected before expiration of a time period ofabout 30 minutes from some start event, such as the initiation of atreatment session (e.g., when the machine is turned on or when the maskpressure first raises above ambient pressure) or by some other resettingor starting of a timer, these detected events would be considered vocalcord closure. Those events that are detected after expiration of thetime period would then be considered treatable upper airway obstruction.In view of the preference to treat upper airway obstruction, rather thanvocal cord closure, an alternative methodology may simply abstain fromdetecting obstructive events prior to the expiration of the time period.In this methodology, all events after the expiration of the time periodare determined to be treatable upper airway obstructive events.

A preferred treatment protocol for responding to obstructive events in amanner that distinguishes between upper airway obstructive events andvocal cord closure is illustrated in FIG. 9. During a period of earlysleep, e.g. about 30 minutes from starting use of the apparatus,obstructive events detected by any method are determined to be vocalcord closure. No increase in pressure from an initial pressure settingis permitted. During the latter stages of sleep, e.g. after about 30minutes, detected obstructive events are treated by a step up in endexpiratory pressure (EEP), for example, about 1-2 cm H₂O, which may befixed or user selectable. Optionally, additional increases in pressurefor detected obstructive events may be limited by the expiration ofadditional time periods. For example, a subsequent step in the EEP upondetection of another obstructive event would not be permitted untilafter about 10 minutes. Furthermore, increases in the EEP would not bepermitted beyond a maximum pressure level. In the preferred embodiment,the initial EEP pressure is 5 cm H₂O and increases in pressure stepabout 2 cm H₂O and these steps are permitted after about 30 minutes haveelapsed from the time that the patient has begun using the mask.Additional steps are permitted at about ten minute intervals thereafter.The limitation imposed by the additional intervals can be phased outafter a sufficient time has elapsed which will substantially suggestthat the patient is fully asleep, e.g., after about 1 to 2 hours ofsleep, preferably after 100 minutes.

While vocal cord closure may also be determined by an insertable cameraproximate to the patient's vocal cords, due to issues of comfort andequipment cost it is preferred to determine an indicator of closure asdescribed above from a measure of respiratory airflow as a function oftime. Of course, additional indicators may be based upon sleep data fromEEG signals (electroencephalography) and/or EOG signals(electro-oculographic) or any other equipment used to determine sleep.Those skilled in the art would understand the nature of data from thesedevices for the purpose of distinguishing between the various stages ofsleep that a subject will experience.

B. Positive Pressure Dose Measure

In order to manage administration of the treatment of the heart in thepresence of positive pressure, for example, by delivering bilevel CPAPtreatment, the preferred device calculates an index that represents thedose of treatment for the heart. With such a dose index, a physician mayprescribe a certain quantity of treatment. Compliance then can bemonitored by the pressure treatment apparatus so that the physician andpatient can be certain that a treatment regimen is being satisfied.

Steps in such a methodology are illustrated in the flow chart of FIG.11. In a delivery step 1102, a supply of breathable gas is delivered ata pressure above atmospheric to the airway of the patient. In a controlstep 1104, the pressure is controlled to perform work of the heart ofthe patient. In a determination step 1106, a heart treatment indexrepresentative of a dose of heart treatment experienced by the patient'sheart is determined.

It is desirable to have such an index as a function of time and afunction of pressure to assess the number of pressure hours received bythe patient. Thus, the index may be viewed as having a pressurecomponent and a time component. The preferred pressure component of theindex is an average value of the pressure. Thus, the average pressuretaken over the time period of treatment multiplied by the length of thetime period can serve as a measure of dose. For example, if the averagepressure delivered to the patient during a 5 hour treatment period is 7cm H₂O, the dose is 35 cm H₂O-hours. In this embodiment of the doesindex, higher treatment pressures are weighted more than lower treatmentpressures for purposes of determining compliance. Thus, a patientreceiving an average 9 cm H₂0 of support pressure would satisfy the 35cm H₂0-hours dose in approximately 3.9 hours.

Due to the breathing patterns experienced by CHF patients (i.e.,Cheyne-Stokes breathing, partial obstruction, etc.) and the likelyswings in pressure that are a result of treatment of such events, theindex may be derived as the root mean square value of the pressuredelivered during the treatment period. Such an index will moreaccurately provide an indicator of the pressure delivered given thepressure swings when compared with, for example, a median pressure.

With regard to the time component of the index, since CHF patients arelikely to experience airway obstruction either due to a typicalobstructive apnea or reflex vocal cord closure, the total time durationthat the device delivers pressure support is not necessarily sufficientto accurately measure the treatment actually experienced by a subject'sheart. Rather, it is preferred to consider the time period during whichpressure is actually delivered to the thorax by excluding periods ofclosed airway obstruction. To this end, the preferred heart failuretreatment dose index is a measure of delivered pressure during a timeperiod that excludes duration of obstructive apneas and reflex vocalcord closures.

Accordingly, in the preferred embodiment, the dose index is a product ofthe average of the delivered pressure and an estimate of the number ofhours that pressure is delivered to the thorax. Thus, the total numberof hours of machine use is reduced to exclude the total time associatedwith airway obstructive events as these events are detected duringtreatment. For this purpose, the device quantifies periods of detectedobstruction from the start of each obstruction through a point when theobstruction is alleviated. For example, an obstruction timer maycommence when an obstructive apnea is detected if a calculated airflowvariance falls below a threshold value. The obstruction timer willcontinue to track time until the calculated variance rises again abovethe threshold value. Of course, the time of the event may be excluded ifit is determined to be a central apnea since the thorax will still betreated during a central apnea. Those skilled in the art will recognizeother methods for quantifying the time period of the obstructive eventsthat prevent pressure treatment of the thorax. Similarly, during theperiod of obstructive apnea, the determination of the average pressuremay be suspended so that the pressure readings during this period do notaffect the average pressure calculation.

Optionally, time periods of high or significant mask leak may similarlybe excluded from the computation of thorax treatment time and averagepressure. Such periods may be considered ineffective treatment. Forexample, the volume of measured airflow taken over a single breath cyclemay be compared to a threshold value to determine if significant or highleak exists. Thus, the time period from such a comparison indicatinghigh leak until the time that the comparison no longer indicated highleak could be excluded from the total treatment time and averagepressure calculation. Those skilled in the art will recognize othermethods for detecting leak for purpose of excluding periods of leak fromthe dose computation.

With such a device, treatment can be prescribed and delivered based onthe dose index. For example, a pressure treatment device may optionallybe implemented to accept as input a prescribed dose as a set-point indexor prescribed threshold before use of the device. Then during treatment,the actual delivered dose is calculated and compared to the prescribedthreshold to assess whether the actual delivered does satisfies theprescribed threshold. When the threshold is reached, the device mayautomatically cease delivery of pressure treatment. Preferably, theextent of the compliance with the prescribed dose may be recorded in thedevice for use by or transmission to the patient's physician. In oneembodiment, the device may indicate that the dose has been achieved bygenerating a message, warning or alarm to the user or physician toadvise the user that the dose has been reached and no further treatmentis required.

C. Cardiac Pressure Oscillations

As previously noted, positive pressure can be applied to a subject'srespiratory system to provide an oscillating force in the thoraxproximate to the wall of the heart to assist with the work of the heart.To this end, positive pressure may be supplied to the subject's lungs tocause compression on the heart by increasing pressure during thesystolic phase when the cardiac muscles contract. By reducing deliveredpressure during the diastolic phase, treatment will more readily allowthe cardiac muscles to relax.

These changes in pressure amplitude are chosen to have some effect onthe heart without causing the subject discomfort. In the preferredembodiment, a cardiac pressure waveform with peak amplitude in thechosen range is superimposed on a level of positive pressure deliveredin accordance with CPAP, hi-level pressure support or other pressuresupport variant such as the pressure delivered in accordance with asmooth pressure waveform template as disclosed in U.S. patentapplication Ser. No. 08/935,785. The cardiac treatment pressure waveformcycles to increase and decrease in phase with the cycle of the heart.One such synchronized cardiac waveform is illustrated in FIG. 4.Preferably, the peak pressure amplitudes associated with the systolicphase are in a range of about 1 to 4 cm H₂0. Although FIG. 4 depicts aclipped sinusoidal waveform, other waveform shapes may be used. Forexample, an oscillatory square wave may also serve to provide supportfor the contractions of the heart.

In order to synchronize the oscillations, the device determines thephase of the heart and triggers the cardiac pressure oscillations. Inthe preferred embodiment, cardiac rhythm is detected from an outputsignal from either an electrocardiogram (ECG) or a set of electrodes.Such electrodes each include a signal wire from the device which isattached to a metallic or otherwise conductive skin contact that candetect an electric charge. Electrical current generated by the heart ina person's chest flows to the surface and at the skin producesdifferences in electrical voltage which can be measured between pairs ofelectrodes placed at two points on the skin. Data from such electrodesis analyzed to time the delivery of each increase in pressure togenerate the cardiac pressure waveform. Alternatively, cardiac rhythmmay be determined from an airflow signal as described in U.S. Pat. No.5,704,345 or otherwise estimated from the patient's pulse by anautomated pulse rate detector on the wrist or finger of the patient.

An example of the superposition of a cardiac pressure waveform and arespiratory pressure level or waveform is illustrated with reference toFIGS. 4, 5 and 6. The waveform shown in FIG. 5 depicts a hypotheticalhi-level respiratory treatment pressure waveform with an IPAP generatedfor the inspiratory portions of a subject's respiratory cycle and anEPAP generated for the expiratory portions of the subject's respiratorycycle. The waveform depicted in the graph of FIG. 6 illustrates thesuperposition of the synchronized cardiac treatment pressure waveform ofFIG. 4 with the respiratory treatment pressure waveform of FIG. 5.

D. Congestive Heart Failure Indicator

Another difficulty involving the treatment of heart failure patientsrelates to disease management. There are currently no known methods foraccurately and continuously assessing a degree of severity or a degreeof change in the patient's condition to assist care providers inpredicting whether the patient is improving or not in response to aparticular treatment regime. For example, when a physician treats heartfailure with a particular drug, there is often insufficient informationconcerning whether the prescribed dose is particularly effective for thepatient.

Accordingly, the preferred device determines or calculates one or moreheart failure indicators or indices to indicate a change in heartfailure condition or to rate a degree of severity of the heart failurecondition. The changing value of such an indicator or index may providea diagnostic tool for the physician to assess the state of the patient'shealth. For example, the indicator may provide information to inform thephysician that the dosage of pharmacological agents given to the patientought to be changed. If the index indicates that heart failure wasstabilizing then it may be appropriate to reduce or maintain the dosagein use. Optionally, in accordance with an assessment of the heartfailure indicator in which the indicator suggests that the patient isdestabilizing, the device may begin to provide a specific dose ofsuperimposed cardiac oscillations to perform some work of the patient'sheart as previously described.

Alternatively, the index may be used to monitor the efficacy oreffectiveness of a drug protocol. For example, the index may bemonitored for a group of patients. This may include the storing ofmultiple indices in a database of patient information. By an analysis ofsuch data, it may be determined that a drug is safe and/or appropriateas a treatment for heart failure in general.

Moreover, an index determined in accordance with the invention may beused by a physician in conjunction with other known methods of analyzingthe health of the patient. For example, an index or indicator inaccordance with the invention may be used in conjunction with changes inweight, medication dosage or lung fluid levels to detect changes in asubject's condition. Such an index may also be part of a battery ofindicators for diagnosing whether or not a patient is suffering fromheart failure in the first instance. For example, levels of BNatriuretic Peptide (BNP), a protein present in the blood that issecreted by heart muscle that is failing, which may be determined by ablood test and recorded by the apparatus and associated with the periodsof use of the device as well as the indicators determined by the device.

Optionally, the apparatus prompts for input from a user so that the usercan enter weight changes medication dosage, number of apneas orhypopneas, or other heart failure monitoring characteristics, such aslevels of BNP. Thus, the device also serves as a database for recordingheart failure monitoring characteristics. The device then mayperiodically transmit such data relating to the patient's condition to acentralized system for physician analysis. Alternatively, transmissionsof such data may be performed on physician prescribed times or intervalsor based on certain event criteria being met, such as a transmissiontrigger based upon recorded data meeting certain thresholds (e.g., thetotal number of apneas or hypopneas exceeding a certain threshold levelor a change in a heart failure indicator compared with prior indices.)

The heart failure indices are determined from an analysis of thepatient's breathing characteristics or by the machines' responses to thepatient's breathing patterns. The indices may be determined inconjunction with a protocol for delivering treatment pressure or withoutsuch treatment pressure, for example, by simply monitoring patientrespiratory airflow. Such indices serve as heart failure indicators toshow patient improvement or relapse as detailed below. Such indicatorsmay be recorded over various sessions with the device. In subsequentsessions, a current indicator or an average of such indicators may becompared with an indicator or average of such indicators from one ormore prior sessions to analyze changes in the indicators.

Steps in such a methodology for evaluating heart failure in a patientare illustrated in the flow chart of FIG. 12. In a delivery step 1202,breathable gas is delivered to the patient at a pressure aboveatmospheric. In a measuring step 1204, the respiratory airflow of thepatient is measured. In a determining or calculating step 1206, a heartfailure indicator representing information about the patient's heartcondition is derived from the respiratory airflow.

One such index is based upon a measure of the extent of the subject'sCheyne-Stokes breathing or a so-called “Cheyne-Stokiness” of thepatient. As previously described, heart failure patients typicallyexperience periods of the waxing and waning defined as Cheyne-Stokesbreathing. The cycle of the waxing and waning typically will vary in therange of about 30 to 90 seconds, or even as high as 120 seconds. Byexamining variations or changes in the cycle, such as periodicity, itcan serve as an indicator of the degree of severity or change in heartfailure condition.

For example, by measuring the number Cheyne-Stokes cycles anddetermining increases or decreases in this number. Alternatively, theenvelope period or duration of each waxing and waning cycle may bemeasured and compared to the time for one or more previous cycles or anaverage of prior cycles. A decrease may suggest that the patient's heartfailure condition is improving. Similarly, an increase may suggest thatthe patient's heart failure condition is destabilizing. Thus, a currentcycle time would be compared to a threshold, e.g., based on previouslydetermined cycle times. Alternatively, the difference between currentand prior measurements or a ratio of current and prior measurement mayalso serve as such an indicator.

One method for determining the duration of Cheyne-Stokes breathing is todetermine the start time of a hyperapnea or hyperventilation and the endtime of a subsequent apnea or hypopnea as shown in respiratory graph ofFIG. 7. In one embodiment, the start of the cycle may be indicated by anincrease in a short-term measure of ventilation, for example, aninstantaneous ventilation or a volume of airflow measured over a periodof several seconds or less. Alternatively, it may be determined from anincrease in peak values of airflow. The duration would then include thetime period of the increase, the time period of a subsequent decrease inthe short-term ventilation measure or decrease in peak airflow, and mayinclude the duration of a possible period of cessation of airflow. Othermethods for detecting each of these events, i.e., hyperapnea orhyperventilation, hypopnea and central apnea, are known in the art.

Alternatively, taking into account the nature of Cheyne-Stokesbreathing, duration may be determined by identifying a particular pointin one cycle and the similar point in the subsequent cycle. One suchmethod involves a measure of instantaneous ventilation and a measure ofrecent average ventilation as disclosed in U.S. patent application Ser.No. 09/316,432. By monitoring a measure of instantaneous ventilationrelative to a measure of recent average ventilation or target based onthe average ventilation measure, an estimate of cycle time forCheyne-Stokes breathing may be determined. For example, by monitoringthe intersections of an instantaneous ventilation and an averageventilation and determining the time between two similar intersectionsor otherwise measuring the time between two such intersections, theduration of a Cheyne-Stokes cycle may be estimated. This method isillustrated in FIG. 7. The figure shows three graphs. The first graph ofrespiratory airflow depicts a number of cycles of Cheyne-Stokesbreathing. The second graph shows a plot of one instantaneousventilation (Instantaneous Ventilation 1) as measured in accordance withU.S. patent application Ser. No. 09/316,432. The third graph is a plotof another measure of instantaneous ventilation (InstantaneousVentilation 2). As shown, the duration of the Cheyne-Stokes cycle on therespiratory graph 70 may be determined by measuring the duration on theventilation graph 72. Thus, a timer may be reset to zero at the momentinstantaneous ventilation changes from less than to greater than therecent average ventilation and the time elapsed is assessed when thatcondition is again true. Alternatively, the time of these points may berecorded and cycle time may be derived by calculation to determine thedifference between the recorded times.

In another embodiment, the heart failure indicator may be an indicationof Cheyne-stokes breathing determined from an average of a flow signaladjusted to remove leak. For example, if a measure of ventilation which,for preference, is an average minute ventilation or inspired volume overa one minute period, exceeds about 15 L/min, it is likely that thepatient is experiencing an episode of Cheyne-Stokes breathing. The totalnumber of these events can be logged. The changing value of such a heartfailure indicators or indices may provide a diagnostic tool for thephysician to assess the state of the patient condition. Thus, theinformation may be recorded over several sessions and increases ordecreases in this number or durations from prior sessions, or averagesfrom prior sessions, may then be determined, recorded and transmitted tothe physician for review and analysis.

In one embodiment, the heart failure indicator may be a measure ofventilation variability. Such a measure may be a ratio of a maximum andminimum of a measure of ventilation. In patients experiencingCheyne-Stokes breathing, a measure of minute ventilation will vary froma low of about 0 L/min to a high of about 25 L/min. For non-CheyneStokes breathing, patients typically will only experience changes inminute ventilation in a range of about 4 to 8 L/min. Accordingly, aheart failure indicator may be the ratio of the minimum to the maximum,preferably, as follows:Heart Failure Indicator=Minimum(Ventilation)/Maximum(Ventilation)

Notably, as the minimum measure of ventilation may be near 0, it ispreferred that the minimum be divided by the maximum to avoid a divideby zero operation. Moreover, as an alternative, the indicator may bebased on the ratio of minimum and maximum tidal volumes (i.e., a measureof ventilation of the patient taken during the inspiratory portion of arespiratory cycle.) For patients experiencing Cheyne-Stokes breathing,the tidal ventilation may vary from 0 up to 1.5 or as much as 2.5liters. Another method for determining an appropriate heart failureindicator, which is intended to assess the extent of Cheyne-Stokesbreathing, is to measure a volume of airflow, e.g., a minute ventilationor a filtered measure of airflow with a time constant of about 10seconds, and record the resulting waveform or sampled data representingthe waveform over time. At the conclusion of a period of measurement,such as one treatment episode or various intervals during one suchepisode, the data or waveform is analyzed to isolate the frequenciestypically associated with Cheyne-Stokes waxing and waning periods. Thisfrequency analysis involves a Fourier Transform such as a DFT or FFT toexamine a frequency range of about 5×10-2 Hz to 1.1×10-2 Hz since theperiod for a typical Cheyne-Stokes cycle is in the range of about 20 to90 seconds in duration. With such an analysis preferably with emphasisin a sub-range of 1/30 Hz to 1/60 Hz, a quantitative measure of theextent of Cheyne-Stokes breathing may be extracted. Since normal patientrespiratory cycles would be reflected at much higher frequencies, thechosen range would be indicative of Cheyne-Stokes cycles. An example ofa frequency spectrum 80 is shown in FIG. 8A, depicting a frequencyanalysis of components of a measure of airflow in the range offrequencies generally associated with Cheyne-Stokes breathing. Thebottom graph of FIG. 8A illustrates the existence of Cheyne-Stokesbreathing in the minute ventilation signal of the top graph of FIG. 8A.The bottom graph of FIG. 8B illustrates the lack of Cheyne-Stokesbreathing in the minute ventilation signal of the top graph of FIG. 8B.

By observing different distributions or different spreads at differentpeaks in the band or by comparing shifting frequency bands in thefrequency range of interest over different periods, changes in theresultant frequency spectrum will indicate changes in Cheyne-Stokesbreathing. Thus, an embodiment of the invention quantifies components inthe range of Cheyne-Stokes frequencies as one or more indices and/orgenerates a visual frequency spectrum of the Cheyne-Stokes frequenciesto show a graphical pattern of the patient's Cheyne-Stokes breathing.Such indicators may be based upon analyses or quantifications of thespectrum that include a determination of a total power, for example, asum of amplitudes or magnitudes of components at all frequencies in theentire band of frequencies or a portion thereof, a determination ofmagnitudes or a measure of a peak value at one or more separatefrequencies, a determination of a measure of central tendency orstandard deviation, or similar.

By monitoring changes in the resulting quantification and/or patternshifts the indicators may then serve as indicators of a change in heartfailure condition. For example, if the magnitudes or patterns indicatethat the patient's Chenye-Stokes breathing is increasing in duration,becoming more frequent or otherwise increasing in intensity, it mayindicate that the patient's heart failure condition is destabilizing anda different treatment approach (medicinal or otherwise) to the patient'scondition may be warranted. Alternatively, a decrease in the duration,frequency or intensity of such breathing may indicate that the currenttreatment approach is appropriate. Thus, these quantifications (e.g.,duration, a magnitude, sum of magnitudes, or standard deviation) derivedduring a current session may be compared to a threshold value. Thethreshold may be a pre-determined acceptable level or a quantificationderived from a previous analysis in a current session or from a previoustreatment session with the device.

In one embodiment of the device based upon an analysis of any of thevarious indicators previously mentioned or as a function of a change insuch indicators, a warning such as an audible alarm or other visualsignal is generated or this information may be transmitted to a systemfor access by a physician. For example, if the current heart failureindicator including a duration is greater than a prior indicator or adesired threshold of change in the indicator, a warning is generated.The alarm or warning is intended to alert a care provider or user to thepotential destabilization of the patient if a change in the indicatorsuggests such destabilization. Alternatively, indicators may be recordedover time to provide long-term statistical analysis of the patient'scondition subsequent to treatment with the device. For example, suchindicators may be recorded over various sessions with the device in adatabase or other information storage medium.

In one embodiment of the invention, the device may respond to the onsetof one or more periods of Cheyne-Stokes breathing by reducing pressuresupport to permit data from a Cheyne-Stokes breathing event to berecorded in the absence any significant treatment pressure that mightimpact or change the nature of the pattern of breathing. For example, ifa Cheyne-Stokes breathing event is detected, the amplitude of pressuresupport may be reduced by about 50% for a particular period of time, forexample, about 10 minutes, so that data during the event may berecorded. A limit on the number of these untreated data collectionsessions may also be implemented so that the patient's Cheyne-Stokesbreathing does not go completely untreated during any treatment sessionwith the apparatus. For example, the device may record data for threeCheyne-Stokes events during any single treatment session. This reducedtreatment data collection process may be performed consecutively foreach consecutively detected Cheyne-Stokes event up to the optionallimit. Alternatively, a certain number of detected Cheyne-Stokes events,one or more, may be fully treated by an appropriate support pressureresponse before a subsequently detected event will be evaluated in areduced treatment data collection process. Thus, a full treatmentprocess and reduced treatment data collection process may alternate overa number of detected Cheyne-Stokes events.

In addition to providing a measure for a qualitative analysis of thepatient's heart failure condition, the indicator may be utilized as aninput to an automated pressure adjustment algorithm that can serve toprovide additional treatment for the heart. For example, the indicatormay be utilized to increase end expiratory pressure. In one embodiment,when the minute ventilation exceeds a 15 L/min threshold, the pressuremay be increased by a small quantity, for example about 1 cm H₂O.Optionally, any additional changes to the treatment pressure based onthe indicator would not take place until expiration of a predeterminedtime period, such as about 10 minutes.

Similarly, in the case of the indicator that is determined as a ratio ofminimum and maximum ventilation measures, based on a comparison of theindicator with a threshold value, the pressure may be increased by asmall degree, e.g., about 1 cm H₂O. For example, the pressure may beincreased as follows: If MIN(tidal volume)/MAX(tidal volume)<a thresholdin a range of about 0.04 to 0.25, preferably about 0.1 to 0.2, forexample, about 0.15, after a period of about 10 minutes then increasethe pressure by about 1 cm H₂O.

The aforementioned indicators or the measures on which they are basedmay be filtered with a time constant of about 5 or 10 minutes. Such anoperation permits the filtering out of the consequences of short termchanges in these continuously determined measures.

While the invention has been described with various alternativeembodiments and features, it is to be understood that the embodimentsand features are merely illustrative of the principles of the invention.Those skilled in the art would understand that other variations can bemade without departing with the spirit and scope of the invention asdefined by the claims.

Where ever it is used, the word “comprising” is to be understood in its“open” sense, that is, in the sense of “including”, and thus not limitedto its “closed” sense, that is the sense of “consisting only of”. Acorresponding meaning is to be attributed to the corresponding words“comprise”, “comprised” and “comprises” where they appear.

It will be understood that the invention disclosed and defined hereinextends to all alternative combinations of two or more of the individualfeatures mentioned or evident from the text or drawings. All of thesedifferent combinations constitute various alternative aspects of theinvention.

The invention claimed is:
 1. An apparatus for treating a patient withheart failure comprising: a blower configured for supplying acontrollable level of breathable gas to a mask at a pressure aboveatmospheric wherein the mask provides a sealable connection with thepatient's airway; a differential pressure transducer configured togenerate a flow signal representative of the patient's airflow; and aprocessor configured to receive said flow signal and control said levelof breathable gas; wherein said apparatus comprises programmed controlinstructions to control a determination of a heart failure indicatorindicative of a degree of heart failure of the patient from said flowsignal, and wherein the apparatus further comprises programmed controlinstructions to analyze said heart failure indicator to determine achange in said heart failure indicator over time, wherein said change isa difference between a previous heart failure indicator and a subsequentheart failure indicator, wherein the apparatus further comprisesprogrammed control instructions to control beginning of subsequent heartfailure treatment based at least in part upon said change in said heartfailure indicator, wherein said subsequent heart failure treatmentcomprises an increase in pressure.
 2. The apparatus of claim 1 whereinsaid determination evaluates an extent of Cheyne-Stokes breathing ofsaid patient.
 3. The apparatus of claim 2 further comprising prog7ammedcontrol instructions that reduce said pressure during a detected episodeof Cheyne-Stokes breathing for a predetermined period of time to permita determination of said heart failure indicator from said predeterminedperiod of time such that a pattern of Cheyne-Stokes breathing can emergewithout significant influence from treatment pressure.
 4. The apparatusof claim 2 wherein said evaluation comprises analyzing said airflow todetermine a duration of a waxing and waning cycle.
 5. The apparatus ofclaim 2 wherein said determination comprises a frequency analysis ofsaid airflow in a range of frequencies indicative of Cheyne-Stokesbreathing.
 6. The apparatus of claim 5 wherein said frequency analysisof said airflow is in a range of about 1/20 hertz to 1/90 hertz.
 7. Theapparatus of claim 6 wherein said heart failure indicator comprises amagnitude of a component of said airflow at a frequency in said range.8. The apparatus of claim 7 wherein said heart failure indicator is asum of magnitudes of components of said airflow in a sub-range offrequencies in said range.
 9. The apparatus of claim 7 comprisingfurther instructions that compare said magnitude with a threshold value.10. The apparatus of claim 9 wherein said threshold value is a magnitudederived from a previous frequency analysis.
 11. The apparatus of claim 5wherein said frequency analysis of said airflow is performed with datasampled from a measure of ventilation derived from said airflow.
 12. Theapparatus of claim 11 wherein said measure of ventilation is a minutevolume.
 13. The apparatus of claim 2 wherein said determinationcomprises measuring a number of Cheyne-Stokes cycles during a treatmentsession.
 14. The apparatus of claim 1 further comprising programmedcontrol instructions that prompt for heart failure monitoringcharacteristics and record said heart failure monitoring characteristicsand said heart failure indicator in a memory.
 15. The apparatus of claim14 wherein one of said heart failure monitoring characteristics is alevel of B natriuretic peptide.
 16. The apparatus of claim 1 furthercomprising programmed control instructions that compare said heartfailure indicator to a prior heart failure indicator determined during aprevious treatment session.
 17. The apparatus of claim 1 furthercomprising programmed control instructions that analyze said heartfailure indicator as a function of a threshold value.
 18. The apparatusof claim 1 wherein said indicator is a function of a measure ofventilation.
 19. The apparatus of claim 18 wherein said indicator is ameasure of ventilation variability.
 20. The apparatus of claim 19wherein said indicator is a ratio of a minimum ventilation and a maximumventilation.
 21. The apparatus of claim 20 wherein the minimumventilation and maximum ventilation are derived from a measure of minuteventilation.
 22. The apparatus of claim 20 wherein the minimumventilation and maximum ventilation are derived from a measure of tidalvolume.
 23. The apparatus of claim 1 wherein programmed controlinstructions analyze said heart failure indicator to determine a furtherchange in said heart failure indicator over time wherein said furtherchange is a ratio of a previous heart failure indicator and a subsequentheart failure indicator.
 24. The apparatus of claim 1 further comprisingprogrammed control instructions that generate a warning signal as afunction of said change.
 25. The apparatus of claim 24 wherein saidwarning signal triggers an audible alarm in said apparatus.
 26. Theapparatus of claim 1 wherein said determination evaluates a number ofoccurrences of central apneas during a treatment session.
 27. A methodof evaluating heart failure condition in a patient comprising steps of:measuring airflow of the patient using a sensor; determining a heartfailure indicator indicative of a degree of heart failure of the patientfrom said airflow; analyzing in a processor said heart failure indicatorto determine a change in said heart failure indicator over time, whereinsaid change is a difference between a previous heart failure indicatorand a subsequent heart failure indicator; and beginning with theprocessor subsequent heart failure treatment based at least in part uponsaid change in said heart failure indicator, wherein said subsequentheart failure treatment comprises increases in pressure.
 28. The methodof claim 27 wherein the determining comprises counting occurrences ofcentral apneas during a treatment session.
 29. The method of claim 27further comprising a step of selecting subsequent heart failuretreatment based at least in part upon the determined change in thepatient's heart failure condition.
 30. An apparatus for treating apatient with heart failure comprising: a blower configured for supplyinga controllable level of breathable gas to a mask at a pressure aboveatmospheric, wherein the mask provides a sealable connection with thepatient's airway; a differential pressure transducer configured togenerate a flow signal representative of the patient's airflow; and aprocessor configured to receive said flow signal and control said levelof breathable gas; wherein said apparatus comprises programmed controlinstructions to control a determination of a heart failure indicatorindicative of a degree of heart failure of the patient, and wherein saidapparatus further comprises programmed control instructions that reducesaid pressure during a detected episode of Cheyne-Stokes breathing for apredetermined period of time to permit a determination of said heartfailure indicator from said predetermined period of time such that apattern of Cheyne-Stokes breathing can emerge without significantinfluence from treatment pressure.
 31. The apparatus of claim 30,wherein the determination is from the apparatus' responses to thepatient's breathing patterns.